Manufacture of alumina.



.80 Drawing.

UNITED STATES PATENT OFFICE.

GILBERT McCULLOCH, OF EAST ST. LOUIS, ILL1NOIS,YASSIGNOR TO ALUMINUMCOM- I 'PANY OF AMERICA, OF NIAGARA FALL S, NEW YORK, A CORPORATION OFPENN- SYLVANIA.

manufacture of alumina, and is applicable to any process of manufacturein which the alumma 1s preclpltated from a solutlon of sodium aluminateby means of carbon dioxid gas. It is applicable, for example, to theprocess described in the patent of Charles.

M. Hall, No. (363,167, dated December 4, 1900. In the practice of suchprocess a solution of sodium aluminate is formed, which may be done byany of the methods known tothe art, and alumina is precipitated from itby passing over it or through it gas containing carbon dioxid. In theresulting reaction the carbon dioxid combines with the soda to formsodium carbonate and precipitates the alumina in the form of aluminumhydrate,

which may be removed with a filter press,

washed, and calcined to form anhydrous alumina which may be employed inthe manufacture of aluminum. It has been found that in such processesthe solution of sodium aluminate which is made frombauxite carries aconsiderable proportlon of silica which in the ordinary processes ofprecipitation by means of carbon dioxid is precipitated along with thealumina and thus contaminates the product. p

I have discovered a process which alumina of much greater purity can beobtained and- I-will now describe the same.

In the practice of my invention I complete the precipitation of aluminafrom the solution of sodium aluminate in the manner' usuall employed, bypassing'over or through the so ution carbon dioxid and agitating thesolution; and I carry on this precipitation until practically all of thealumina is precipitated, which causes also the precipitation ofthesilica from the solution, so that the precipitate is, for thepurposes of alumina, impure. I then add to the solution an amount ofcaustic soda (NaOH), preferably equivalent to about 10 per cent. of theI entlre amount of soda present in the solution reckoned as causticsoda; but the amount may be varied and may be more or less, ac-

cording to the proportion of silica present in Specification of LettersPatent.

Application filed May 19, 1909. Serial No. 496,962.

MANUFACTURE or ALUMIN'A,

Patented Oct. 26, 1909.

the precipitate. The-flow of ()0 gas having been discontinued, I thencontinue to agitate the charge, preferably from one to three hours, inlike manner as is ordinarily practiced during the original precipitationof the alumina, except that during this time no CO gas is introduced. Bythese means, namely, by the actionof the added caustic soda and theagitation of the solution, I find that the eat bulk of the silica whichhas been precipitated with the alumina is re-dissolved in the solutionand that very little of the alumina is re-dissolved with it; so that inthe practice of the process which I have conducted I have found thatfrom a liter, only 1.5 to 3 grams is re-dissolved, but that nearly allof the silica is re-dissolved. At the end of this time, when analysisshows that the silica contained in the precipitate has been mainlyre-dissolved in the solution, I run the entire charge out of theagitators and filter it so as-to remove the alumina; and the filteredliquor containin the re-dissolved silica and a small portion of aluminais then preferably conducted back to the digester of re-dissolvinganother charge of alumina from crude bauxite.

The following is an example of thejn'ocess as it has been practiced byme and which I. give by way of example,-not for the purpose of limitingthe invention to the precise details of procedure or proportion thereinset forth, these being capable of variation by those skilled in the art,within the principle of my invention as dcfined in the claims.

I have taken an original liquor to'be charged into the agitator,containing approximately the following compounds in solution: A1 0,, 75to 85 grins. per 'L., Na. ,CO,, 40 to 50 grins. perf L., Na,O combinedwith Alf), and reckoned as NaOH 105 to 115 grins. per liter, SiO .15 to.30 grms. per L. A liquor of this composition having been charged intothe agitator and the'CO gas having been passed thereover and .the liquorbeing agitated, the CO reacts with the sodium 'aluminate forming Na.,(l() and aluminum hydrate until all the alumina and SiO, has beenprecipitated. The liquor at this point has approximately the followingcomposition: Al. ,O .,-OySiQ 0, M 00, 165 to 185 grams per Ia, and theprecipitated aluminum hydrate contains from 10% to 20% precipitate of 75to 85 grams of alumina per department, where it can beused as the basisSiO At this point I cut oil the gas'and add to the agitator an amount ofcaustic soda equal to approximately 10% of the entire soda content andagitate from 1 to 3 hours. I then discontinue the agitation andfilter'the product when the composition of the liquor has been changedto the following: A1 0 1.50 to 3.00 grms. per L., Nil- 00;, 1651.0 L85grins. per L., Na O combined with A1 0 and reckoned as NaOI-I 10 to 20grms. per L., S10 .10 to".20 grins. per L., and the silica in thealuminum. hydrate has been reduced to-,.03% to .067 \(Vith solutionscontaining less silica 1 add a smaller amount of caustic soda, and witha higher silica content in the liquor it is necessary to add morecaustic soda.

1 claim:

1. In the manufacture of alumina by the alkaline process, wherein thealumina is precipitated from a solution of sodium aluminate, theimprovement which consists in adding to the solution after the period ofpreci itation caustic alkali and re-dissolving the silica precipitatedwith the alumina.

2. In the manufacture of alumina by the alkaline process, wherein thealumina is precipitated from a solution of sodium aluminate, theimprovement which consists in adding tothe solution after the period ofprecipitation cai'1stic alkali, re-di ssolving the silica recipltatedwith the alum1naand withdrawing the liquor containing the dissolvedsilica and alumina and using it as":

a basis for dissolving other alumina from bauxite.

3. In the manufacture of alumina by the alkaline process, wherein thealumina is preeipitated froin a solution of sodium aluminate by C0 gas,the improvement which consists in adding to the solution after theperiod of precipitation caustic alkali and redissolving the silicaprecipitated with the alumina.

In testimony whereof, I have hereunto set my hand.

GILBERT MQCULLOCH.

\Vitnesses: I

RAY G'Aime'rr, IRENE BACON.

